Waterproof manifold solids
(An answer, because comment do not allow for enough characters)
If you import the .stl In Blender, it should be easy to close it up in Edit Mode. Best way is probably to do it manually by selecting elements (vertices for instance) and pressing F to create faces between them.
You can probably select all vertices on one side an close it up with a singe press of the F key. However, since you don't want the hole in the center to be closed up (by either gear shape) as well, you should first create a bridge (of a face or an edge) between the inner and outer surface (for either side), before you do that.
It might also be worth knowing that Blender ships with Add-ons that allow for the easy creation of gears and cogwheels (by pressing Shift + a, selecting the desired object, and then clicking the properties in the bottom left corner or pressing F6). There is the Add Mesh: Extra Objects add-on which allows to create Gear objects and adjust their properties before they are added. And there is the Add Curve: Extra Objects which allows for a Cogwheel to be created; basically the same thing but as a Curve Profile.
Both add-ons can be easily activated by checking their box, if you go to the Preferences &gt; Add-ons and search for &quot;extra&quot;.